The invention relates to a transistor amplifier circuit, and more particularly a transistor amplifier circuit in which a pair of circuits are used each including a source follower type field effect transistor in the input stage and a bipolar transistor in the succeeding stage and wherein the pair of bipolar transistors comprise a differential amplifier.
As shown by an amplifier circuit A shown in FIG. 1, in a prior art amplifier circuit of this type, the input stage is constituted by a source follower circuit including a pair of N channel field effect transistors Q.sub.11 and Q.sub.12 and resistors 11 and 12 and the succeeding stage is constituted by a differential amplifier circuit including a pair of PNP type bipolar transistors Q.sub.13 and Q.sub.14, resistors 13 and 14 and a constant current circuit CC.sub.11, in which the gate electrodes of the field effect transistors Q.sub.11 and Q.sub.12 are used as the input terminals, while the collector electrodes of the bipolar transistors Q.sub.13 and Q.sub.14 are used as the output terminals, of the amplifier circuit A. By this construction, the advantages of the field effect transistors and of the bipolar transistors can be manifested namely, the advantage of high input impedance caused by the use of the field effect transistors and the advantage of high gain caused by the use of bipolar transistors can be preserved.
With the circuit construction described above, however, since the dynamic range of the input stage is determined by the voltage across the drain and source electrodes of each field effect transistor the drain-source voltage must exceed a minimum value, which is relatively high. When the field effect transistors are driven by a relatively high voltage, their loss becomes high and the gate leakage current also increases, thus increasing the power loss of the amplifier circuit. For this reason, when this circuit is used as a DC amplifier, a relatively large loss caused by over-heating produces a drift of the operating current in each field effect transistor as the temperature varies. Large gate leakage current results in small input impedance of the field effect transistor so that it is impossible to use a plurality of parallel connected field effect transistors for the purpose of increasing the gain. In addition, in this circuit as the drain-gate voltage in the field effect transistor varies in accordance with the amplitude of the input signal, the capacitance between the drain and gate electrodes and the gate leakage current of the field effect transistor also vary in accordance with the amplitude. In other words, the input impedance on the side of the gate electrode varies thereby causing distortion of the signal. Even when this circuit is incorporated into a negative feedback amplifier the distortion due to the change of input impedance of the amplifier can not be improved by negative feedback.